Obstructions in some patient's airways during sleep can cause limited airflow, leading to apnoea, hypopnoea or snoring. The obstruction is often a collapsed pharynx. The obstruction may be a partial airway obstruction, leading to altered characteristics of the airflow. A hypopnea is a reduction of the flow that is greater than fifty percent, but not complete. An apnea, however, is complete cessation of airflow. Each of these conditions frequently leads to sleep deprivation and other associated health problems.
It is well known to treat patient suffering from sleep deprivation with positive airway pressure therapy (“PAP”). This therapy can be Continuous Positive Airway Pressure (“CPAP”), Variable Positive Airway Pressure (“VPAP”), Bi-level Positive Airway Pressure (“BiPAP”), Auto titrating Positive Airway Pressure (“APAP”) or any numerous other forms of respiratory therapy. The application of positive pressure to the patient's pharynx helps minimize or prevent this collapse. Positive airway pressure therapy is currently supplied by means of an apparatus containing a pressure source, typically a blower, through a tube to an interface or mask, which the patient wears while sleeping.
Control the applied pressure is an important consideration in PAP. Too little pressure tends not to solve the problem. Too much pressure tends to cause discomfort to the patient, such as drying out of the mouth and pharynx, as well as difficulty in exhaling against the applied pressure.
One solution to this problem is to use a bi-level devices which tracks pressure in accordance with a patient's inspiration and expiration. However, an accurate indication of the change from inspiration to expiration is necessary.
It is also desirable to be able to adjust the applied pressure without requiring the patient to attend a sleep centre, so the apparatus will allow in-home adjustments, preferably automatically. One method generally thought to be effective is to monitor the patient to try to anticipate the onset of an obstructed airway, and to adjust the pressure in response. When an elevated upper airway resistance or flow obstruction is anticipated or underway, the apparatus increases the applied pressure. When the patient returns to normal sleep, the applied pressure is reduced.
For bi-level control of an apparatus, that is, matching of pressure with a patient's inspiratory and expiratory breathing flows, the air pressure is supplied at a higher pressure during inspiration and at a lower pressure during expiration. This makes it easier for the patient to breath against the lower pressure during exhalation.
A patient's respiratory flow when measured in the absence of an applied pressure or flow clearly reveals respiratory phases and the transition between expiration and inspiration. In FIG. 1 inspiration is that part of the waveform above the axis and expiration corresponds to that area below the axis. Where the waveform crosses zero indicates the transition points between the phases. That is, the transition point is where the patient changes from inspiration to expiration, or vice versa.
When a patient is breathing through a mask and is supplied with positive airway pressure the air flow waveform that may be recorded is displaced away from the axis by the application of an external flow, as shown FIG. 2, for example. It is difficult to extract inspiratory and expiratory phases and transitions accurately where there is a displacement and without reference to the flow signal itself. Usually the flow signal itself does not provide an obvious indicator of the transition between expiration and inspiration and vice versa. The waveform, for example, does not cross zero at the inspiration/expiration nor at expiration/inspiration transitions.